SiC—C/C composite material, uses thereof, and method for producing the same

ABSTRACT

SiC—C/C composite materials having a suitable kinetic coefficient of friction, good corrosion resistance in strongly oxidizing and corrosive environments, good creep resistance and spalling resistance, and high hardness. The materials are hardly oxidized or abraded even when exposed to high temperatures, while maintaining the excellent impact resistance and light weight of C/C composites. Molten metal pumps using the materials are provided from which components do not dissolve into the molten metal even when used in molten metal and have sufficient thermal impact resistance and oxidation resistance.

CROSS REFERENCE TO RELATED APPLICATION

This application is a division of U.S. application Ser. No. 09/496,377,filed Feb. 2, 2000, now U.S. Pat. No. 6,355,206 B1, the entirety ofwhich is incorporated herein by reference.

BACKGROUND OF THE INVENTION AND RELATED ART

1. Field of the Invention

The present invention relates a novel SiC—C/C composite material usableas jigs for molten metal requiring oxidation resistance at hightemperatures, such as molten metal carrying pumps used in molten metalor molten metal pumps for removing dross, grinding members requiringoxidation resistance at high temperatures, sliding members such asrolling bearings and plain bearings used in apparatuses for makingsemiconductors, precision instruments, cars and aircraft parts, andbraking members used as friction materials for brake disks fitted so asto gear with speed control devices used for stopping or controlling ofspeed of mass-transportation means such as large cars, ultra-high speedtrains, and aircraft. The invention further relates to a method forproducing the composite material.

2. Related Art Statement

Aluminum-zinc alloys, SUS and other various alloys are used as jigs formolten metal, but since they are used at high temperatures, the lifethereof is short, namely, about one week. Therefore, they must befrequently changed, and the changing operation at high temperature isvery difficult. Thus, demanded are those materials which do not givesubstances contaminating the metal in the molten metal and are high inabrasion resistance, oxidation resistance and endurance, and can be usedfor a long period of time at high temperatures.

For example, plating of steel sheets for cars is carried out by dippingthe steel sheets to be plated in metals such as zinc and aluminum moltenby heating (molten metal). However, when the materials to be plated arerepeatedly dipped, impurities in the molten metal grow in the form ofparticles to cause formation of solid suspended materials (dross) in themolten metal. If the plating is continued as it is, plating thicknessbecomes uneven or appearance of the plated articles becomes poor.

Therefore, in metal plating step, the plating operation is carried outwith removing dross by a molten metal pump. FIG. 9 shows an example ofthe molten metal pump. In FIG. 9, steel sheet 11 which is a material tobe plated is plated by dipping it in molten metal 13 by means of pulley12 in a hydrogen atmosphere. Molten metal pump 14 is generally providedwith dross storage portion 15 and dross passage 16 having openings atboth ends. One end opening of dross passage 16 communicates with moltenmetal 13 outside the dross storage portion 15 and another end opening ofdross passage 16 communicates with molten metal 13 inside the drossstorage portion 15. Furthermore, the dross passage 16 has impeller 18for bringing about a liquid flow from one end side to another end sidefitted to revolving shaft 17 in another side of the dross passage. Inthe molten metal pump 14 shown in FIG. 9, the inner space of innercontainer 20 forms the dross storage portion 15 and the space betweenthe inner wall of outer container 21 and outer wall of the innercontainer 20 forms the dross passage 16.

Sliding materials such as rolling bearings and sliding bearings arewidely used in various fields such as semiconductors, ceramics,electronic parts and manufacture of vehicles as constitutive members ofapparatuses for making semiconductors, precision instruments, cars andaircraft parts. Especially, at present, with the rapid progress oftechnical innovation, sliding materials used for sliding bearings,sliders, bearing holders, etc. in the fields of space development suchas shuttle spacecraft and spaceplanes, and fields of energy such asnuclear energy, solar energy and hydrogen energy, are used at hightemperatures of higher than 400° C., at which oil cannot be used as alubricant owing to burning or carbonization, or at low temperatures atwhich oil freezes. Therefore, sliding materials per se must have akinetic coefficient of friction as small as possible and must hardly beworn. Furthermore, naturally, these sliding materials are required tohave high strength and high reliability (tenacity and impact resistance)at moderate to high temperatures (200-2000° C.), and environmentalresistance (corrosion resistance, oxidation resistance and radiationresistance). Moreover, due to the recent demand for energy savings,sliding materials are also required to be light in weight so that theycan be driven by small loading.

Under the circumstances, silicon nitride or silicon carbide materialswhich are excellent in heat resistance and high in strength havehitherto been used as sliding materials, but these have a great kineticcoefficient of friction of 0.5-1.0 and are apt to cause wear of othermaterials and are not necessarily suitable as sliding materials. Inaddition, they are high in density and consume great energy for drivingand have a large kinetic coefficient of friction. Moreover, they arebrittle per se, and are considerably brittle if flawed and, moreover,have insufficient strength against thermal and mechanical shocks.

As a means for solving these defects, ceramic composite materials (CMC)comprising composites of continuous ceramic fibers have been developedand used as sliding materials. These materials are high in strength andtenacity even at high temperatures and have excellent impact resistanceand environmental resistance, and, thus, they are being studied mostlyin Europe and America as main refractory sliding materials.

On the other hand, as friction materials used in braking devices fittedto mass-transportation means such as large cars, ultra-high speedtrains, and aircraft, carbon fibers-in-carbon often called C/Ccomposites, which are very high in friction coefficient at hightemperatures and light in weight, are widely used at present. In thesemass-transportation means, it is common to continue braking for a longtime depending on changes in driving circumstances or to repeatedlybrake at high frequency. Therefore, in the case of braking devices usingC/C composites as friction materials, the friction materials are exposedto high temperatures in the air for a long time. Accordingly, sincefriction materials using C/C composites are basically mainly composed ofcarbon fibers which readily burn at high temperatures, the carbon fibersreact with oxygen under such conditions of being exposed to hightemperatures for a long time to cause considerable wear or emission ofsmoke, leading to serious accidents. However, from the points offriction force at high temperatures and flexibility needed in fitting todisk brakes, substitutes therefor have not yet been discovered.

On the other hand, ceramic type SiC—C/C composite materials (CMC)comprising a composite of a ceramic matrix and fibers which areincorporated in the matrix have been developed in the following manner.That is, several hundred to several thousand ceramic long fibers ofabout 10 μm in diameter are bundled to form a fiber bundle (yam), thesefiber bundles are arranged in planar or three-dimensional directions toform unidirectional sheets (UD sheets) or various cloths, or thesesheets or cloths are laminated, thereby forming a preform (fiberpreform) of a given shape, and a matrix is formed in the preform by aCVI method (chemical vapor impregnation method) or an inorganic polymerimpregnating and firing method or by filling the preform with ceramicpowders by a cast molding method and then firing the preform to form amatrix.

As examples of CMC, there have been known C/C composites comprisingcarbon fibers arranged in planar or a three-dimensional directionbetween which matrices comprising carbon are formed, and SiCfiber-reinforced Si—SiC composites formed by impregnating a molded bodycomprising SiC fibers and SiC particles with metallic silicon. Moreover,British Patent No.1457757 discloses a composite material obtained byimpregnating a C/C composite with metallic silicon to form SiC. In thiscomposite material, a very common material is used as the C/C compositeforming a skeletal part. That is, a phenolic resin as a binder is coatedon carbon fibers of a suitable thickness, they are laminated in auniform fiber direction so as to give the desired shape, and thelaminate is put in a mold having a specific shape, followed bycompressing and curing to obtain a molded body of C/C composite. This isfired and the fired body is impregnated with metallic silicon. Thephenolic resin is carbonized by the firing, but the amount of theremaining carbon is at most about 50%, and after firing, many fine poresare present randomly around the carbon fibers. This is impregnated withmetallic silicon, but it is very difficult owing to the random presenceof the pores to uniformly impregnate the whole of the fired body withsilicon. An SiC matrix is formed by the reaction of free carbon producedby carbonization of the phenolic resin used as a binder with metallicsilicon used for impregnation, but, in this case, owing to the porespresent randomly, the matrix does not become homogeneous and,simultaneously, metallic silicon also randomly reacts with carbon fibersto form an SiC layer on the carbon fibers. As a result, there is aproblem in that the carbon fibers become short at the portion of the SiClayer being formed in at least a part of the composite material,resulting in deterioration of impact resistance, flexural strength, highlubrication and abrasion resistance.

Since C/C composites are high in tenacity, they are superior as brakingmembers because of their excellent impact resistance, light weight andhigh hardness, but they are composed of carbon and hence cannot be usedat high temperatures in the Presence of oxygen and have a limit in theuse as refractory sliding materials. Moreover, since they are relativelylow in hardness and in compression strength, abrasion wear is large whenused as sliding materials or braking members.

On the other hand, SiC fiber-reinforced Si—SiC composites are excellentin oxidation resistance and creep resistance, but are apt to be flawedon the fiber surface. Moreover, SiC fibers are high in wettability withSi—SiC and these are firmly bonded and, hence, drawing effect betweenthe mother body and the fibers is small. Thus, they are inferior to C/Ccomposites in tenacity, and therefore low in impact resistance and arenot suitable for sliding materials having complicated shapes or having athin wall portion, such as bearings and sliders. Furthermore, they lackreliability for materials usable for a long time as jigs for moltenmetal used at high temperatures. Thus, materials high in reliability andusable for a long period of time have not yet been provided.

That is, in the above-mentioned molten metal pump 14, the members whichcontact with molten metal 13 of high temperature of 500-800° C., such asouter container 21, inner container 20, revolving shaft 17 and impeller18, must be made of materials having impact resistance. Furthermore, ifthe material constituting the molten metal pump bleeds into the moltenmetal, quality of the plated articles is affected, and so there must beused materials which do not bleed out at high temperatures. Moreover,the materials constituting the molten metal pump must have oxidationresistance because they are sometimes used in the air. From theseviewpoints, SIALON is used as the material of the outer container, etc.which contact with molten metal among the members constituting themolten metal pump.

SUMMARY OF THE INVENTION

An object of the present invention is to provide a novel SiC—C/Ccomposite material which has a kinetic coefficient of friction within arange which does not damage sliding properties, has abrasion resistance,is light in weight, is excellent in impact resistance, creep resistanceand spalling resistance, is high in hardness, and is hardly oxidized orworn even when exposed to high temperatures in the presence of oxygen.The material maintains the excellent impact resistance and light weightof C/C composites and is free from the defects of C/C composites whichare used now as braking materials of mass-transportation means (i.e.,changing operation at high frequency because of considerable wear whichunavoidably occurs in the presence of oxygen caused by hightemperatures). SIALON now used for the portions of molten metal pumpswhich require oxidation resistance has no problem in oxidationresistance, but is inferior in thermal impact resistance, and, forexample, when it is used at 800° C. for 100 hours, cracks occur at theportions in the vicinity of the surface of the molten metal.

The molten metal pump which constitutes one aspect of the presentinvention has been accomplished under the above circumstances, and thusanother object of the present invention is to provide a molten metalpump from which the components do not dissolve out when used in moltenmetal and which has sufficient thermal impact resistance and oxidationresistance. Still another object is to provide a novel SiC—C/C compositematerial which has high endurance usable as jigs for molten metal usedat high temperatures of higher than 600° C., preferably higher than 800°C.

As a result of intensive research conducted by the inventors forattaining the above objects, it has been found that an SiC—C/C compositematerial comprising silicon carbide, carbon fibers and a carboncomponent other than carbon fibers and having a structure comprising askeletal part and a matrix formed around the skeletal part, in which atleast 50% of silicon carbide is of β type, the skeletal part is formedof carbon fibers and a carbon component other than carbon fibers,silicon carbide may be present in a part of the skeletal part, thematrix is formed of silicon carbide, the matrix and the skeletal partare integrally formed, and the composite material has a porosity of0.5-5% and a two-peak type distribution of average pore diameter, can beused for making molten metal jigs, especially, molten metal pumps,thereby to be able to attain the above objects for the followingreasons. Firstly, said SiC—C/C composite material is also excellent inoxidation resistance, creep resistance and spalling resistance and canalso be used as sliding materials under such conditions that lubricantscannot be used due to high temperature conditions, in the presence ofoxygen. Secondly, it exhibits excellent impact resistance and lightweight as friction materials for brakes, shows sufficient abrasionresistance even in the presence of oxygen when used as frictionmaterials for disk brakes which unavoidably generate high temperature,and does not require changing operation at high frequency as requiredfor C/C composites and can be used continuously. Thirdly, it does notrelease components which contaminate the molten metal and has sufficientimpact resistance and oxidation resistance. Thus, the present inventionhas been accomplished.

That is, the present invention provides an SiC—C/C composite materialcomprising silicon carbide, carbon fibers and a carbon component otherthan carbon fibers and having a structure comprising a skeletal part anda matrix formed around the skeletal part, characterized in that at least50% of the silicon carbide is of β type, the skeletal part is formed ofcarbon fibers and a carbon component other than carbon fibers, siliconcarbide may be present in a part of the skeletal part, the matrix isformed of silicon carbide, the matrix and the skeletal part areintegrally formed, and the composite material has a porosity of 0.5-5%and a two-peak type distribution of average pore diameter.

Furthermore, the present invention provides a molten metal pump in whichat least the portions which contact with molten metal are composed ofthe above SiC—C/C composite material. The molten metal pump according toanother aspect of the present invention is preferably a molten metalpump which has a dross storage portion and a dross passage havingopenings at both ends and in which one end opening of the dross passagecommunicates with molten metal only at the surface portion of the moltenmetal outside the dross storage portion, another end opening of thedross passage communicates with molten metal only at the surface portionof the molten metal inside the dross storage portion, said dross passageis formed of a space defined by the inner wall of the outer containerand the outer wall of the inner container constituting the dross storageportion, and the dross passage has an impeller on another end side whichis fitted to a revolving shaft and brings about liquid flow from one endside and another end side, characterized in that at least the portionswhich contact with molten metal comprise the above SiC—C/C compositematerial. Furthermore, the molten metal pump is preferably one in whichthe portions which contact with the molten metal are the dross passage,the impeller and the revolving shaft. Moreover, the molten metal pumpmay be one which is used for molten zinc or molten aluminum.

The present invention further provides a method for producing theSiC—C/C composite material which comprises a step of keeping metallicsilicon and a molded body comprising C/C composite or a C/C compositefired body in a furnace at a furnace inner temperature of 1100-1400° C.and under a furnace inner pressure of 0.1-10 hPa for 1 hour or more withflowing an inert gas in an amount of 0.1 NL or more per 1 kg of totalweight of the molded body or the fired body and the metallic silicon,thereby reacting the carbon component constituting the matrix of the C/Ccomposite with the metallic silicon to form a matrix comprising siliconcarbide, a step of raising the furnace inner temperature to 1450-2500°C. with keeping the furnace inner pressure as it is thereby melting andimpregnating the metallic silicon into open pores of the molded body orfired body to grow silicon carbide and simultaneously sufficientlyfilling the remaining pores with the metallic silicon, and a step ofincreasing the furnace inner pressure to about 1 atm. with once reducingthe furnace inner temperature to environmental temperature or keepingthe furnace inner temperature as it is, and raising the furnace innertemperature to 2000-2800° C., whereby the produced silicon carbide ormetallic silicon filled in the pores is diffused from the matrix intothe C/C composite composed of carbon fibers and a carbon component otherthan carbon fibers and is reacted with the carbon.

The SiC—C/C composite material of the present invention basicallycomprises 20-80% by weight of carbon and 80-20% by weight of siliconcarbide, and the matrix comprising an SiC material is formed integrallybetween yarn assemblies comprising carbon fibers combinedthree-dimensionally and integrated so that they are not separated. Evenif about 0.3% by weight of metallic silicon remains per total weight ofthe composite material, this gives substantially no influence to theperformance of the composite material of the present invention. Asmentioned hereinafter, when a layer of matrix formed of SiC material isprovided, thickness thereof is preferably at least 0.01 mm, morepreferably at least 0.05 mm, further preferably at least 0.1 mm.

Furthermore, in the novel SiC—C/C composite material of the presentinvention, it is preferred that the matrix has such a composition thatcontent of silicon increases in inclined manner in proportion to thedistance from the yarn. Moreover, the SiC—C/C composite material maycontain at least one substance selected from the group consisting ofboron nitride, boron, copper, bismuth, titanium, chromium, tungsten andmolybdenum. In addition, it is preferred that the SiC—C/C compositematerial has a kinetic coefficient of friction of 0.05-0.6 at roomtemperature and a porosity of 0.5-5%.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an oblique view which schematically shows the skeletal part ofthe SiC—C/C composite material of the present invention.

FIG. 2(a) is a cross-sectional view taken along the line IIa—IIa in FIG.1, and FIG. 2(b) is a cross-sectional view taken along the line IIb—IIbin FIG. 1.

FIG. 3 is a partial enlargement of FIG. 2(a).

FIG. 4 is a partial sectional oblique view schematically showing theessential part of the SiC—C/C composite material of the presentinvention.

FIG. 5 is a chart showing the relation between temperature and decreasein weight.

FIG. 6 is a photograph showing the state of small protrusions protrudingfrom the surface of a test piece.

FIG. 7 is a photograph showing the phase structure of the section of thesame test piece.

FIG. 8 is an enlarged photograph of one example of the open pore formedin the matrix in FIG. 7.

FIG. 9 is a schematic sectional view showing a general construction ofthe molten metal pump.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The SiC—C/C composite material of the present invention is formed of acomposite material comprising ceramics and carbon which is provided witha skeletal part comprising a C/C composite and a Matrix layer comprisingan SiC material.

The novel SiC—C/C composite material of the present invention will beexplained below.

This is a material of new concept which comprises a novel C/C compositeimproved in its basic construction. In this specification, “C/Ccomposite” means a molded body or a fired body obtained by firing themolded body, and the molded body is obtained in the following manner.Carbon fiber bundles are prepared by containing therein a pitch, a cokeor the like as a powdery binder which acts as a matrix of carbon fiberbundles and becomes free carbon after firing and, if necessary,additionally a phenolic resin powder or the like and a flexible filmcomprising a plastic such as thermoplastic resin is formed around thecarbon fiber bundles to obtain preformed yarns as flexible intermediatematerial. The preformed yarns are made into a sheet-like material by themethod disclosed in JP-A-2-80639, a desired number of the sheets arelaminated and the laminate is molded by a hot press to obtain a moldedbody.

The C/C composite used as a basic material can be produced in thefollowing manner. Usually several hundred to several tens of thousandsof carbon fibers of about 10 μm in diameter are bundled to form a fiberbundle (yam). This fiber bundle is covered with a thermoplastic resin toobtain a flexible intermediate material. This is made into a sheet-likematerial by the method disclosed in JP-A-2-80639, and the resultingsheet-like materials are arranged in planar or three-dimensionaldirections to obtain unidirectional sheets (UD sheets) or variouscloths, or the sheets or cloths are laminated to form a preform of agiven shape (fiber preform). The film of a thermoplastic resin or thelike formed on the outer surface of the fiber bundles of the preform isfired to carbonize and remove the film. The disclosure of JP-A-2-80639is incorporated herein by reference. In the C/C composite used in thepresent invention, the carbon component other than the carbon fibers inthe yarn is preferably a carbon powder, especially preferably agraphitized carbon powder.

In the SiC—C/C composite material of the present invention, a C/Ccomposite composed of carbon fiber bundles is used as a skeletal part,and, therefore, even if SiC is formed in a part thereof, the structureof carbon fibers is retained without being broken and the length of thecarbon fibers is not reduced by the silicon carbide. Thus, mechanicalstrength possessed by the C/C composite is nearly retained or increasedby the silicon carbide. This is a great characteristic. Besides, thematerial has a composite structure in which a matrix comprising an SiCmaterial is formed between the adjacent yams in the yam assembly.

The SiC material in the present invention is a material comprisingsilicon carbide differing in the degree of bonding to carbon, and thisSiC material is produced in the following manner. In the presentinvention, C/C composite is impregnated with metallic silicon, and inthis case the metallic silicon reacts with carbon atoms constituting thecarbon fibers in the composite and/or free carbon atoms remaining on thesurface of the carbon fibers to cause partial carbonization of themetallic silicon. Therefore, partially carbonized silicon is produced onthe outermost surface of the C/C composite or between the yarnscomprising carbon fibers, and thus a matrix comprising silicon carbideis formed between the yarns.

This matrix can contain some different phases from a siliconcarbide-based phase, in which a very slight amount of silicon and carbonbond to each other, to a pure silicon carbide crystal phase. However,this matrix contains metallic silicon in an amount less than the limitof detection (0.3% by weight) by X-ray analysis. That is, this matrixtypically comprises a silicon carbide phase, but the silicon carbidephase can contain a SiC-based phase in which the content of siliconchanges in an inclined manner. Therefore, the SiC material is a genericname of SiC series materials in which carbon is contained in the rangeof 0.01-50 mol % in concentration. Control of the carbon concentrationto less than 0.01 mol % is not practical because strict metering of theamount of the added metallic silicon is required in relation with theamount of free carbon in the C/C composite, and control of temperatureat the final step mentioned hereinafter becomes complicated. However, itis theoretically possible to control the carbon concentration to about0.001 mol %.

In the SiC—C/C composite material of the present invention, it ispreferred that the material has a silicon carbide phase produced alongthe surface of the yarns and, in addition, a part of the matrixprotrudes from the surface as small protrusions as shown in FIG. 6. Thisis especially preferred in the case of using as brake members, grindingmembers, etc. because the surface roughness is larger. In the SiC—C/Ccomposite material, silicon carbide phase is formed between yarns.Therefore, the surface of the yarns is strengthened by the siliconcarbide phase. Moreover, in the central portion of the matrix, there areformed pores of relatively large pore diameter of about 100 μm as amedian, and, hence, microscopic stress dispersion occurs due todeformation of the pore portion depending on the stress applied.

Further, this SiC—C/C composite material preferably has a matrix havingsuch inclined composition as the content of silicon increasing inproportion to the distance from the surface of the yarn. Moreover, inthe SiC—C/C composite material, preferably the yarn assembly comprisingcarbon fibers is formed of a plurality of yarn array elements, and eachof the yarn array elements is formed by two-dimensionally arrangingyarns comprising a bundle of a specific number of carbon fibers innearly parallel with each other, and the yarn assembly is constructed bylaminating the yarn array elements. In this way, the SiC—C/C compositematerial has a laminate structure in which yarn array elements of aplurality of layers are laminated in specific directions.

In this case, it is especially preferred that the longer directions ofthe yarns in the adjacent yarn array elements cross each other. Thereby,dispersion of stress is further accelerated. The longer directions ofthe yarns in the adjacent yarn array elements especially preferablycross at right angles with each other. Furthermore, preferably, thematrix is continuous in the SiC—C/C composite material to form athree-dimensional network structure in the composite material. In thiscase, it is especially preferred that the matrices are two-dimensionallyarranged in nearly parallel with each other in the respective yarn arrayelements, and the matrices produced in the adjacent yarn array elementsis continuous to each other, whereby the matrices form athree-dimensional lattice. Moreover, the space between the adjacentyarns may be completely filled with the matrix, but only a part of thespace may be filled with the matrix.

The SiC—C/C composite material of the present invention comprises askeletal part composed of a C/C composite having a three-dimensionalstructure comprising a yarn assembly formed by laminating yarn arrayelements in which a specific number of yarns comprising carbon fiberbundles are arranged and an SiC material formed in the form ofthree-dimensional lattice as matrices between the yarns constituting theskeletal part. The SiC—C/C composite material of the present inventionhas a kinetic coefficient of friction in the range of 0.05-0.6 at roomtemperature, and furthermore the low oxidation resistance of C/Ccomposite can be overcome by providing on the surface a matrix layer ofSiC material having oxidation resistance, creep resistance and spallingresistance. Thus, the SiC—C/C composite material can be used as slidingmaterials, members for braking and members for molten metal which areunavoidably exposed to high temperatures in the presence of oxygen.Since the porosity is controlled to 0.5-5%, variation of kineticcoefficient of friction caused by change of environment is very small,and stable braking performance can be exhibited. Abrasion wear at hightemperatures is preferably 1.0%/hour or less, more preferably 0.6%/houror less at 500° C. Moreover, the material has abrasion resistanceresulting from the excellent abrasion resistance inherently possessed bysilicon carbide.

Furthermore, since the SiC—C/C composite material has as the skeletalpart a C/C composite basically composed of a yam assembly comprisingcarbon fiber bundles, it is light in weight and meets the demand forenergy savings. Especially, since the carbon fibers do not become shortfibers after formation of the matrix as already mentioned above, themechanical strength is maintained, and since the longer directions ofthe fibers of the respective yam array elements cross each other,preferably cross at right angles with each other in the yam assembly, noanisotropy in shape occurs. The matrix comprising free carbon formed inthe skeletal part has high uniformity. Therefore, in the SiC—C/Ccomposite material of the present invention produced by impregnationwith metallic silicon, the metallic silicon uniformly disperses andreacts with carbon, and hence the composition of the constitutingsubstances in a specific volume is uniform. Since the composition isuniform, internal stress is evenly distributed. Therefore, deformationhardly occurs by sintering, and large molded articles of complicatedshape, especially, large and thin-wall molded articles of complicatedshape can be produced.

Moreover, since the skeletal part comprises C/C composite, the materialis high in tenacity, excellent in impact resistance and high inhardness. Accordingly, the defect of low high-temperature abrasionresistance of the conventional C/C composites can be overcome withretaining the characteristics of the C/C composites. Further, since theC/C composites have continuous open pores, SiC as a matrix formed byimpregnating metallic silicon through the pores forms a continuousstructure and a three-dimensional network structure. Therefore, anyportions cut out have higher abrasion resistance than the C/C compositewhich is the skeletal part, and besides the high heat dissipation andflexibility inherently possessed by the C/C composite can be maintained.

In the case of the molten metal pump which is another aspect of thepresent invention, at least the portions which contact with molten metalare made of the SiC—C/C composite material to impart sufficient thermalimpact resistance and oxidation resistance to the molten metal pump and,simultaneously, inhibit dissolution of the components of the material ofthe molten metal pump into the molten metal. That is, the SiC—C/Ccomposite material has excellent strength and thermal impact resistancenot only at room temperature (20° C.), but also at high temperatures andis excellent in oxidation resistance and thus it gives the abovecharacteristics to the molten metal pump. Especially, the material hasthe characteristic that thermal impact resistance is markedly higherthan that of SIALON.

The molten metal pump of the present invention has no limitation insystem and kind, and can be any pump which can be dipped in molten metalin use. For example, as shown in FIG. 9, a molten metal pump used forremoving dross in molten metal generally has a dross storage portion anda dross passage having openings at both ends, one end opening of thedross passage communicating with surface part of the molten metaloutside the dross storage portion and another end opening communicatingwith the surface part of the molten metal inside the dross storageportion. Furthermore, the dross passage has a means for causing liquidflow from one end side to another end side of the dross passage, such asan impeller fitted to a revolving shaft.

As shown in FIG. 9, a more specific embodiment of the above molten metalpump is such that the molten metal pump 14 is formed of at least aninner container 20, an outer container 21, a revolving shaft 17, animpeller 18 and a revolving shaft-driving part 24, the inner space ofthe inner container 20 forms a dross storage portion 15, and the spacebetween the inner wall of the outer container 21 and the outer wall ofthe inner container forms a dross passage 16. In this case, the memberswhich contact with molten metal 13 in use of the molten metal 14, suchas inner container 20, outer container 21, revolving shaft 17 andimpeller 18, are made of the above SiC—C/C composite material. Othermembers such as the revolving shaft-driving part 24 may also be made ofthis material. The kind of the molten metal in which the molten metalpump of the present invention is used has no limitation, and the pumpcan be suitably used in molten metals such as zinc, aluminum, iron, tinand copper, but the pump can be especially suitably used for molten zincor molten aluminum when the molten metal temperature is taken intoconsideration.

The C/C composite in the present invention is a material comprisingtwo-dimensionally or three-dimensionally arranged carbon fibers betweenwhich a matrix comprising carbon is formed as aforementioned, and thismay contain elements other than carbon, such as boronnitride, boron,copper, bismuth, titanium, chromium, tungsten and molybdenum as far as10-70% of carbon fibers are contained.

In the case of using the composite material having a matrix layercomprising SiC material on the surface, the SiC material melts to formglass, and the speed of protecting the skeletal part against oxygen ishigher than the speed of diffusion of oxygen into the skeletal part.Therefore, the case can be avoided where the C/C composite used as theskeletal part is oxidized with the diffused oxygen, and the skeletalpart can be protected from oxidation. Accordingly, the SiC—C/C compositematerial of the present invention shows self-restoration property andcan be used for a long period of time. This effect is also obtained evenwhen the matrix contains the third components such as boron nitride,copper and bismuth mentioned above.

Furthermore, since the SiC material is larger in thermal expansioncoefficient than the C/C composite, when the SiC material is merelycoated on the surface of the skeletal part, the layer comprising the SiCmaterial readily peels off from the skeletal part due to the differenceof thermal expansion coefficient in the use of long term under oxidizingcondition at high temperatures while, in the present invention, the SiCmaterial is formed integrally as a matrix layer of the SiC—C/C compositematerial, whereby strength of the fibers in the direction of laminationis increased and the peeling off can be inhibited and thus the materialhas excellent properties as sliding materials and braking members.

The SiC—C/C composite material of the present invention will beexplained in more detail referring to the drawings.

FIG. 1 is an oblique view which schematically explains the skeletal partof the SiC—C/C composite material of the present invention, FIG. 2(a) isa cross-sectional view of the SiC—C/C composite material of the presentinvention taken along the line IIa—IIa in FIG. 1, and FIG. 2(b) is across-sectional view of the SiC—C/C composite material of the presentinvention taken along the line IIb—IIb in FIG. 1. FIG. 3 is a partialenlargement of FIG. 2(a).

The skeletal part of SiC—C/C composite material 7 comprises yarnassembly 6. The yarn assembly 6 is formed by laminating, in verticaldirection, yarns array elements 1A, 1B, 1C, 1D, 1E and 1F. In each yarnarray element, yarns 3 are two-dimensionally arranged and the longerdirections of the yarns are nearly in parallel with each other. Thelonger directions of the yarns in the yarn array elements adjacent toeach other in vertical direction (upper and lower direction) cross atright angles with each other. The longer directions of yarns 2A in theyarn array elements 1A, 1C and 1E are parallel with each other and crossat right angles with the longer directions of yarns 2B in yarn arrayelements 1B, 1D and 1F. Each yarn comprises fiber bundle 3 composed ofcarbon fibers and carbon component other than the carbon fibers. Theyarn assembly 6 in the form of three-dimensional lattice is formed bylaminating the yarn array elements. Each yarn is flattened in the stepof pressure molding mentioned later and is in nearly elliptic form.

In each of yarn array elements 1A, 1C and 1E, matrix 8A is filledbetween the adjacent yarns, and the matrix 8A extends along the surfaceof yarn 2A in parallel to the yarn 2A. In each of yarn array elements1B, 1D and 1F, matrix 8B is filled between the adjacent yarns, and thematrix 8B extends along the surface of yarn 2B in parallel thereto.

As shown in FIG. 2(a), FIG. 2(b) and FIG. 3, matrices 8A and 8B comprisesilicon carbide phase 4 which covers the surface of the yarns. A part ofthe silicon carbide phase may protrude from the surface as smallprotrusion 9 or may protrude into the carbon fiber layer inside thecomposite material. In this small protrusion, there are formed pores(voids) 5 having a pore diameter of about 100 μm as a median. Since mostof the small protrusions 9 are formed along traces of the matrixcomprising carbon component other than carbon fibers of the startingmaterial C/C composite, density of the small protrusions 9 per unit areacan be adjusted by suitably selecting the distance between the yarnsand/or the distance between the yarn array elements. The silicon carbidephase 4 may also be formed between the adjacent yarn 2A and yarn 2B.

Each of the matrices 8A and 8B extends along the surface of the yarnnarrowly, preferably, linearly, and the matrix 8A and the matrix 8Bcross at right angles with each other. The matrix 8A in the yarn arrayelements 1A, 1C and 1E and the matrix 8B in the yarn array elements 1B,1D and 1F which cross at right angles with 1A, 1C and 1E continue toeach other at boundary between yarns 2A and 2B. As a result, thematrices 8A and 8B form a three-dimensional lattice as a whole.

FIG. 4 is a partial sectional oblique view which schematically shows apart of the essential portion of the SiC—C/C composite material as anembodiment of the present invention. In this embodiment, matrices 8A and8B are respectively formed between the adjacent yarns 2A and between theadjacent yarns 2B in the yarn array elements. The matrices 8A and 8Bhave the silicon carbide phase 4 formed in contact with the surface ofthe yarns 2A and 2B, respectively.

It is preferred that the SiC material phase has such an inclinedcomposition as the carbon concentration decreases in proportion to thedistance from the surface of the yarn.

As braking members and grinding members, it is preferred that thesurface of the SiC—C/C composite material is silicon carbide phase andsmall protrusions 9 comprising silicon carbide are formed protruding ata relatively high density.

Thickness of the matrix layer formed by impregnating the skeletal partwith SiC material is preferably at least 0.01 mm, more preferably atleast 0.05 mm, further preferably at least 0.1 mm. If the thickness ofthe matrix layer is less than 0.01 mm, endurance required, for example,for sliding materials under high oxidizing conditions cannot beobtained.

Moreover, concentration of silicon bonded to carbon in the matrix layerof the SiC—C/C composite material of the present invention preferablydecreases inwardly from the surface toward the inner portion.

By giving an inclination to the silicon concentration in the matrixlayer, corrosion resistance and strength, and healing function fordefects in the surface layer part and inner layer part in stronglyoxidative corrosion environment can be highly improved, and, besides,thermal stress deterioration of the material due to the difference inthermal expansion coefficient can be inhibited. This is because thesilicon concentration in the surface layer part is relatively higherthan in the inner layer part, and, hence, micro-cracks generated arehealed during heating to maintain oxidation resistance.

Furthermore, the C/C composite used in the SiC—C/C composite material ofthe present invention may contain at least one substance selected fromthe group consisting of boron nitride, boron, copper, bismuth, titanium,chromium, tungsten and molybdenum.

Since these substances have lubricity, when these are contained in theskeletal part comprising C/C composite, lubricity of the fibers in theskeletal part impregnated with SiC material can be maintained anddecrease of tenacity can be inhibited.

For example, content of boron nitride is preferably 0.1-40% by weightbased on 100% by weight of the skeletal part comprising the C/Ccomposite. If the content is less than 0.1% by weight, the effect ofimparting lubricity by boron nitride cannot be sufficiently obtained,and if it is more than 40% by weight, the brittleness of boron nitrideappears also in the SiC—C/C composite material.

The SiC—C/C composite materials of the present invention explained abovepossess together the impact resistance, high hardness and light weightof the C/C composite and the oxidation resistance, spalling resistance,self-lubricity and abrasion resistance of the SiC material, and, inaddition, have self-restoration properties. Therefore, they can standthe long term use under oxidizing conditions at high temperatures, andcan be suitably used as sliding materials and braking members. Inaddition, members for molten metal, especially molten metal pumps madeof the composite material which constitute another aspect of the presentinvention are excellent in thermal impact resistance and oxidationresistance and, besides, do not release the components which contaminatethe molten metal. Thus, the molten metal pumps can be said to bemarkedly superior pumps.

The SiC—C/C composite material of the present invention can be producedpreferably by the following method, which is one aspect of the presentinvention.

That is, a powdery binder which acts as matrix and finally becomes freecarbon, such as pitch or coke and, if necessary, a phenolic resin powderare incorporated in a bundle of carbon fibers, whereby carbon fiberbundles are prepared. A flexible film comprising a plastic such asthermoplastic resin is formed around carbon fiber bundles as disclosedin JP-A-2-80639 to obtain preformed yarns which are flexibleintermediate materials. The preformed yarns are made to a sheet-likeprepreg sheet, and a desired number of the sheets are laminated,followed by molding the laminate by a hot press under the conditions of300-2000° C. and normal pressure to 500 kg/cm² to obtain a molded body.If necessary, this molded body is carbonized at 700-1200° C. andgraphitized at 1500-3000° C. to obtain a fired body.

The carbon fibers may be either pitch carbon fibers obtained bypreparing a pitch for spinning from petroleum pitch or coal tar pitch,melt spinning the pitch, rendering the spun fibers infusible andcarbonizing the fibers or PAN carbon fibers obtained by carbonizingacrylonitrile (co)polymer fibers.

As carbon precursors necessary for the formation of matrix, there may beused thermosetting resins such as phenolic resin and epoxy resin, tar,pitch, etc. and these may contain cokes, metals, metal compounds,inorganic and organic compounds, etc.

Then, the molded body or the fired body prepared as mentioned above andsilicon carbide are kept in a furnace at 1100-1400° C., under an innerpressure of 0.1-10 hPa for 1 hour or more. The keeping time is variabledepending on various factors, and can be such as being sufficient toremove, from the firing atmosphere, gases such as CO generated withchanging of inorganic polymers or inorganic materials to ceramics andsufficient to inhibit external contamination of the firing atmospherewith O₂ in the air. Furthermore, in this case, it is preferred to forman SiC layer on the surface of the molded body or the fired body withflowing an inert gas in an amount of 0.1 NL (normal litter:corresponding to 5065 liters in the case of 1200° C. and a pressure of0.1 hPa) or more per 1 kg of the total weight of the molded body or thefired body and silicon. Then, the temperature is raised to 1450-2500°C., preferably 1700-1800° C. to melt and impregnate silicon into openpores of the molded body or the fired body to first form the SiCmaterial.

Then, the furnace inner temperature is once reduced to the environmentaltemperature (20-25° C.) or the furnace inner pressure is increased toabout 1 atm with keeping the inner temperature as it is, and the innertemperature is raised to 2000-2800° C., preferably 2100-2500° C.,whereby metallic silicon which may remain and the already producedsilicon carbide are diffused into the carbon fibers and the carboncomponent other than carbon fibers (which has the same meaning as freecarbon containing partially graphitized carbon) and are reacted withthese carbons. About 1 hour is sufficient as the keeping time in thiscase. In case a molded body comprising C/C composite is used in thisprocess, firing of the molded body is also conducted to producesimultaneously an SiC—C/C composite material.

It is preferred that the molded body or the fired body and siliconcarbide are kept at 1100-1400° C., under an inner pressure of 0.1-10 hPafor 1 hour or more, and in this case, it is desirable to control theinert gas to flow in an amount of 0.1 NL or more, preferably 1 NL ormore, further preferably 10 NL or more per 1 kg of the total weight ofthe molded body or the fired body and silicon.

In this way, by carrying out the firing (namely, the stage beforemelting and impregnation of metallic silicon) in an inert gasatmosphere, the gas such as CO generated with changing of inorganicpolymers or inorganic materials to ceramics is removed from the firingatmosphere and the external contamination of the firing atmosphere withO₂ in the air is inhibited, and, as a result, porosity of the compositematerial obtained by subsequent melting and impregnation of metallicsilicon can be maintained at lower level.

Furthermore, when the molten metallic silicon is molten and impregnatedin the molded body or the fired body, the atmospheric temperature israised to 1450-2500° C., preferably 1700-1800° C. In this case, theinner pressure of the firing furnace is preferably 0.1-10 hPa. Then,reaction of silicon including the metallic silicon with carbon iscompleted, and the furnace inner pressure is increased to about 1 atm.and the furnace inner temperature is raised to 2000-2800° C., preferably2100-2500° C. in order to diffuse silicon carbide into carbon fibers andthe carbon component other than the carbon fibers. The raising oftemperature may be started after cooling to room temperature or may bestarted as it is. By this heat treatment at high temperature undernormal pressure, the metallic silicon completely disappears. Thus, evenin the case of using as a molten metal pump, no metallic silicondissolves out into the metal in the molten metal. Furthermore, siliconcarbide formed by the heat treatment under reduced pressure diffusesinto the carbon fibers and the carbon component other than the carbonfibers to form SiC material and simultaneously pores of large porediameter are formed inside the matrix from which silicon has beenremoved.

Thus, there are formed two kinds of pores, namely, those of relativelysmall pore diameter produced by the heat treatment under reducedpressure and those of relatively large pore diameter formed by heatingat high temperature and under normal pressure. That is, an SiC—C/Ccomposite material having two-peak type distribution of pore diameter isobtained. Due to the presence of the two kinds of pores which appear asmicro-pores on the surface when the material is processed to make amember, roughness of the surface becomes great and kinetic coefficientof friction is larger than that of starting material C/C composite.Therefore, when the composite material is used as braking members, thehigher braking effect is exhibited in addition to the effects ofimprovement of oxidation resistance and mechanical strength caused bythe formation of SiC as the matrix.

Since the phenomenon of expansion in the longer direction of the C/Ccomposite at the time of firing is inhibited in the reaction offormation of silicon carbide, precision of size in the longer directionof molded body can be enhanced. Therefore, a large-sized thin-wallmolded article which cannot be produced using silicon carbide materialscan be easily produced using the SiC—C/C composite material of thepresent invention. Especially, since the SiC—C/C composite material ofthe present invention is high in oxidation resistance and excellent inphysical characteristics including hardness, flexural modulus, tensilemodulus, etch, large-sized thin-wall members which are required to havehigh temperature resistance and oxidation resistance and which cannotconventionally be produced can be relatively easily produced. The degreeof formation of silicon carbide can be continuously changed bycontrolling the amount of metallic silicon used, the heating temperatureunder reduced pressure, and the heating temperature under normalpressure. Therefore, the method can be said to be an excellent method inwhich hardness, gas permeability, Young's modulus, thermal conductivityand thermal expansion coefficient can be changed in specific ranges inaccordance with the characteristics required in the fields of use.

As mentioned above, when a flexible intermediate material comprising anorganic material is used on the outer surface of the carbon fiberbundles and combined with impregnation and melting of silicon, theflexible intermediate material undergoes thermal decomposition in themolded body or fired body to leave long and narrow open pores in thespaces between the yarns, and silicon easily penetrates into the deepportion of the fired body or molded body through the open pores. Duringthis penetration, silicon reacts with carbon of the yarns to graduallycarbonize the yarn from the surface side, and as a result, the SiC—C/Ccomposite material used in the present invention can be produced.Depending on use, the SiC—C/C composite material having the aboveconstruction may be formed as so-called SiC—C/C composite material layerin only a part of the surface layer portion of the skeletal partcomprising the C/C composite.

Adjustment of the depth of the matrix layer is carried out by adjustingthe open pore content and pore diameter thereof in the molded body orfired body. For example, in the case of adjusting the thickness of theSiC material layer to 0.01-10 mm, at least the open pore content in thevicinity of the surface of the molded body or fired body is adjusted to5-50% and average pore diameter is adjusted to 1 μm or larger. The openpore content in the molded body or fired body is preferably 10-50%, andthe average pore diameter is preferably 10 μm or larger. If the openpore content is less than 5%, the binder in the molded body or firedbody cannot be completely removed, and if it is more than 50%, the SiCmaterial impregnates deeply into the skeletal part to causedeterioration of impact resistance of the composite material.

In order to form the SiC—C/C composite material layer only in thesurface of the C/C composite, it is preferred to use a molded bodyadjusted so that the open pore content at least in the vicinity of thesurface is 0.1-30% during the firing. That is, this can be attained byadjusting thickness of the film of the flexible intermediate materialcomprising heat-decomposable organic material on the carbon fiberbundles.

In order that the open pore content in the molded body or the fired bodyis lowered from the surface toward the inner portion, a plurality ofpreformed sheets comprising preformed yarns differing in binder pitchare arranged so that the binder pitch increases from the inside towardthe surface side, followed by molding them.

Furthermore, when an inclination is given to the silicon concentrationin the SiC—C/C composite material layer, the composite material isproduced using a fired body adjusted so that the open pore content inthe vicinity of the surface decreases from the surface toward the insideor a molded body adjusted so that the open pore content at least in thevicinity of the surface decreases from the surface toward the insideduring the firing. Control of the porosity of the SiC—C/C compositematerial to 0.5-5% can be easily attained by adjusting the amount ofmetallic silicon depending on the open pore content of the molded bodyor fired body in impregnating the molded body or the fired body withmetallic silicon.

Sliding materials, braking members, molten metal jigs, etc. can beproduced from the novel SiC—C/C composite material by cutting thecomposite material to a suitable size and subjecting it to surfacegrinding by a surface grinding machine.

For making a large-sized member of a specific shape, first the C/Ccomposite is molded into a desired shape, and with or without firing it,a matrix comprising silicon carbide is formed in the presence of metalsilicon by a virtue of the method for the production of the SiC—C/Ccomposite material of the present invention. The SiC—C/C compositematerial of the present invention can be suitably used as slidingmaterials and braking members, especially for sliding materials andbraking members of mass-transportation means, and, further, as moltenmetal jigs used at high temperatures.

EXAMPLES

The present invention will be explained in more detail by the followingnon-limitative examples.

Density of the SiC—C/C composite material of the present invention wasmeasured by Archimedean method, and hardness was measured in accordancewith JIS K7202, and other characteristics were evaluated by thefollowing methods.

Measurement of Open Pore Content

Open pore content(%)=[(W₃−W₁)/(W₃−W₂)]×100 (according to Archimedeanmethod).

Dry weight (W₁): The sample was dried in an oven at 100° C. for 1 hourand then weighed.

Weight of the sample in water (W₂): The sample was boiled to completelyfill the open pores with water and weighed in water.

Weight of the hydrous sample (W₃): The sample the open pores of whichwere completely filled with water was weighed in the air.

Measurement of Flexural Strength

A test piece of 10 mm×50 mm×2 mm in size was subjected to three-pointflexing test, and the flexural strength was calculated by the followingformula.

Flexural strength=3/2F_(MAX) L/b h ²

(wherein b denotes width of the test piece, F_(MAX) denotes maximumload, h denotes height of the test piece, and L denotes distance betweenthe supports).

Measurement of Flexural Modulus

The test piece was subjected to three-point flexing test with thedistance between the supports being 40 times the height h of the testpiece, and the flexural modulus was calculated by the following formulausing the initial gradient P/σof the linear part of the load-deflectioncurve.

Flexural modulus=1/4L ³ /b h ³ P/σ

(wherein b denotes width of the test piece, h denotes height of the testpiece, and L denotes distance between the supports).

Measurement of Tensile Strength

A test piece of 9.5 mm×100 mm×6 mm in size, 6.25 mm in thickness ofcentral part, 28.5 mm in length of the central part, and 38 mm incurvature radius of both ends and central part was subjected to tensileloading, and the maximum load just before breaking was obtained. Thetensile strength was obtained by dividing the maximum load by sectionalarea of the test piece.

Measurement of Tensile Modulus

A test piece having the same size as in the tensile strength test waspulled, and the tensile modulus was obtained by dividing the tensilestress by the strain generated in the test piece.

Measurement of Compression Strength

A compression load was applied to the test piece and the compressionstrength was calculated by the following formula.

Compression strength=P/A

(wherein P denotes the load at maximum loading and A denotes the minimumsectional area of the test piece).

Measurement of Interlaminar Shear Strength

The test piece was subjected to three-point flexing test with thedistance between the supports being 4 times the thickness h of the testpiece, and the interlaminar shear strength was calculated by thefollowing formula.

Interlaminar shear strength=3P/4bh

(wherein P denotes the maximum flexing load at break, b denotes width ofthe test piece, and h denotes height of the test piece).

Measurement of Shear Strength

A load was applied to a test piece of 10 mm×25 mm×6 mm in size, and theload when the test piece was broken was taken as the maximum load. Thismaximum load was divided by the area of the test piece to calculate thebreaking load.

Measurement of IZOD Impact Strength

A notched test piece of 10 mm×100 mm×6 mm in size was mounted on atesting stand, and an impact was applied to the notched side by a hammerto break the test piece. The impact strength was calculated by dividingthe absorption energy by the sectional area of the notched portion ofthe test piece.

Measurement of Kinetic Coefficient of Friction

A test piece was set at a jig and rotated at 100 rpm for 10 minutes, andanother material (SUJ 10 mm ball) was pressed to the test piece at aload Fp(N) of 2 kg, and friction force Fs(N) at that time was measured.The kinetic coefficient of friction was calculated by the followingformula.

Coefficient of friction μ=Fs/Fp

Measurement of Temperature at Which Weight Reduced by 5%

Change of weight of the sample was measured with applying sufficientstream in the air and with raising the temperature at a rate of 10°C./min, and the temperature at which the weight of the sample reduced by5% was obtained.

EXAMPLE

1. Production of SiC—C/C Composite Material of the Present Invention

A powdery binder pitch which acts as a matrix for bundles of carbonfibers and finally becomes free carbon for the bundles of carbon fiberswas included in carbon fibers arranged in one direction, and, further,phenolic resin powder and others were contained therein to preparecarbon fiber bundles. A flexible film comprising a plastic such as athermoplastic resin was formed around the resulting carbon fiber bundlesto obtain preformed yarns which were flexible intermediate materials.The resulting preformed yarns are formed into a sheet by the methoddisclosed in JP-A-2-80639, and a necessary number of the sheets werelaminated so that the directions of the carbon fibers in adjacent sheetscross at right angles. The resin of the laminate was cured by a hotpress at 180° C. and under 10 kg/cm². Then, the laminate was fired at2000° C. in nitrogen to obtain a C/C composite having a thickness of 10mm. The resulting C/C composite had a density of 1.0 g/cm³ and an openpore content of 50%.

Then, the resulting C/C composite was put in a carbon crucible filledwith metallic silicon powders of 99.8% in purity and 1 mm in averageparticle size in an amount enough to give a porosity of 5%. Then, thecarbon crucible was transferred into a firing furnace. The C/C compositewas treated under the conditions of a temperature in the firing furnaceof 1300° C., a flow rate of argon gas as an inert gas of 20 NL/min, apressure in the furnace of 1 hPa, and a holding time of 4 hours.Thereafter, the temperature in the furnace was raised to 1600° C. withkeeping the pressure in the furnace as it was, thereby to impregnate theC/C composite with metallic silicon to obtain an SiC—C/C compositematerial having a porosity of 5%.

Density, porosity, interlaminar shear strength, compression strength,flexural modulus, etc. of the resulting SiC—C/C composite material weremeasured and the results are shown in Table 1 in comparison with thoseof the C/C composite used as a skeletal part. As can be seen from theresults shown in Table 1, there are recognized remarkable increase ofhardness and improvement of flexural modulus, tensile strength,compression strength, shear strength, etc. as compared with the C/Ccomposite which is a comparative example, and, on the other hand,decrease in flexing strength, IZOD impact strength, etc. is seen owingto the brittleness inherently possessed by SiC, but the decrease inthese properties is within the tolerance range, taking intoconsideration the remarkable improvement of oxidation resistance.

Furthermore, FIG. 6 is a secondary electron image photograph showing thesurface state of the test piece. FIG. 7 is a reflecting electron imagephotograph showing the sectional structure of the test piece. FIG. 8 isan enlarged photograph of FIG. 7. It can be seen from FIGS. 6-8 that apart of the produced SiC protruded from the surface as fine protrusion9. It can also be seen that this protrusion was formed along the matrixof the C/C composite. It can also be seen that voids, namely, pores wereformed in the protrusion. It is a characteristic that the pores includetwo kinds of pores, namely, large pores and small pores. The large poreshad a median of pore diameter of about 100 μm, and the formation thereofcan be clearly recognized in the enlarged photograph shown in FIG. 8.The small pores had a small median of about 0.5 μm, and presence thereofcannot be directly recognized in FIG. 8.

However, when the composite material was processed as a braking member,the surface roughness increased because the fine pores appearing on thesurface comprised two kinds of pores differing in diameter. That is,since the surface roughness was greater than that of the C/C compositeand kinetic coefficient of friction increased, and as a result, thebraking effect was improved. The content of metallic silicon was 40%.

A test piece was cut out from the C/C composite surface layer part inwhich the SiC material and the C/C composite were sufficientlyintegrated in the above SiC—C/C composite material, and the test piecewas subjected to cutting process into a size of 60 mm in length, 60 mmin width, 5 mm in thickness by a surface grinding machine and then tosurface grinding by #800 grinding stone to make a sliding material.Surface roughness of the ground surface of the resulting slidingmaterial was Ra=1 μm, and flatness was 2 μm in straightness. This wassubjected to performance test as a sliding material to find that thiswas especially excellent in oxidation resistance at high temperatures ascompared with one produced from the C/C composite.

TABLE 1 Example Comparative Example Impregnation rate 40 0 of Si (%)Density (g/cm³) 2.23 1.67 Hardness HRP 51.5 Collapse Porosity (%) 4.4315 Flexural Strength 10.7 24.5 (kgf/mm²) Flexural Modulus 4771 3394(kgf/mm²) Tensile Strength 4.59 Impossible to (kgf/mm²) Measure due toBreaking at chunk Part Tensile modulus 7808 3619 (kgf/mm²) Compression25 13.1 Strength (kgf/mm²) Interlaminar Shear 1.42 1.36 Strength(kgf/mm²) Shear Strength 2.48 1.72 (kgf/mm²) IZOD strength 7.7 28.2(kgf/mm²) Kinetic coefficient 0.238 0.146 of friction

It can be seen from Table 1 that the composite material of the presentinvention composed of a skeletal part comprising a C/C composite and amatrix comprising an SiC material formed around the skeletal part had agreater kinetic coefficient of friction as compared with the C/Ccomposite used as a skeletal part.

Furthermore, the composite material of the present invention wassuperior to the C/C composite in flexural modulus, tensile strength,tensile modulus, compression strength, and shear strength, and wasnearly the same as the C/C composite in interlaminar shear strength.Although it was inferior to the C/C composite in flexing strength,tensile strength and IZOD impact strength, this was practicallyacceptable in the uses requiring high oxidation resistance, such assliding materials, braking members and members used in molten metal.

2. Molten Metal Pump

Removal of dross from molten aluminum was carried out using a moltenmetal pump made of the above SiC—C/C composite material, and occurrenceof cracks in the members constituting the molten pump, oxidation thereofand dissolution of silicon into the melt were examined.

As shown in FIG. 9, molten aluminum 13 was for plating of steel sheets,and had a temperature of 750° C. Furthermore, as shown in FIG. 9, moltenmetal pump 14 was composed of inner container 20, outer container 21,revolving shaft 17, impeller 18 and revolving shaft driving part 24, andthe inner space of the inner container 20 formed dross storage portion15, and the space between the inner wall of the outer container 21 andthe outer wall of the inner container 20 formed dross passage 16. Ofthese members, the inner container 20, the outer container 21, therevolving shaft 17 and the impeller 18 were made of the above material.

The molten metal pump 14 was dipped in the molten aluminum 13, andremoval of dross was carried out for 100 hours with revolving theimpeller 18, and, thereafter, occurrence of cracks in the membersconstituting the molten pump, oxidation thereof and dissolution ofsilicon into the melt were examined.

The occurrence of cracks was visually examined on the members of innercontainer, outer container, revolving shaft and impeller. As theindicator for the degree of oxidation, reduction of weight of thesemembers was measured. Oxidation resistance was evaluated by thefollowing criteria. When the weight reduction was less than 5%, this isindicated by ◯, when it was 5% or more and less than 10%, this isindicated by Δ, and when it was 10% or more, this is indicated by X. Thedissolution of silicon into the melt was judged by examining the changein composition of the test piece. The results were indicated as follows.When presence of silicon in aluminum was confirmed by X-rays, this isindicated by “yes”, and when the presence of silicon was not recognized,this is indicated by “no”. As a result, no cracks occurred, there werenaturally no problems in oxidation resistance, and no dissolution ofsilicon into the melt occurred.

On the other hand, a molten metal pump in which the inner container, theouter container, the revolving shaft and the impeller were made ofSIALON was prepared in the same manner as above, and removal of dross inmolten aluminum was carried out using the molten metal pump, andexamination was conducted on occurrence of cracks in the membersconstituting the pump, oxidation thereof and dissolution of silicon intothe melt. There were no problems in oxidation resistance and dissolutionof silicon, but occurrence of cracks was clearly seen after use for 100hours.

In the case of the molten metal pump made of SIALON, degree of oxidationwas small, but occurrence of cracks was seen in the outer container inthe vicinity of the surface of the molten aluminum. On the other hand,in the case of the molten metal pump of the present invention, no cracksoccurred in all of the members of the inner container, the outercontainer, the revolving shaft and the impeller, and, besides, degree ofthe oxidation was low. Of course, dissolution of silicon into the moltenmetal was not seen.

When the novel SiC—C/C composite materials of the present invention areused as sliding materials, they have a kinetic coefficient of frictionin a suitable range of 0.05-0.5, and a matrix is formed of an SiCmaterial having oxidation resistance, creep resistance and spallingresistance, and thus the low oxidation resistance of C/C composite canbe overcome, and, therefore, even in the presence of oxygen, the slidingmaterials can be used at high temperatures at which oil cannot be usedas a lubricant. Moreover, as shown in FIG. 5, the composite materialshave a high temperature of 757° C. at which the weight reduces by 5%,said temperature being an indicator for oxidation characteristics, andhave high high-temperature resistance characteristics. Therefore, thecomposite materials are useful as sliding materials used under thecondition exposed to high temperature. Furthermore, since a C/Ccomposite is employed as a skeletal part, the composite materials arelight in weight and small in kinetic coefficient of friction and, hence,small in loss of energy and meet the demand for energy saving. Moreover,since the skeletal part is C/C composite, the composite materials arehigh in tenacity, hardness and impact resistance. Accordingly, thedefect of low impact resistance of SiC fiber-reinforced SiC compositematerials can be overcome, and the composite materials can also be usedas sliding material having complicated shape or thin-wall portions.

When the novel SiC—C/C composite materials of the present invention areused as braking members, since they are highly excellent in abrasionresistance under high temperature conditions in the presence of oxygen,and a layer comprising an SiC material having oxidation resistance,creep resistance and spalling resistance is provided on the surface, thelow oxidation resistance of C/C composite can be overcome, and thesliding materials can be used at high temperatures and in the presenceof oxygen. Furthermore, the sliding materials have excellent abrasionresistance together. Especially, since the range of fluctuation ofkinetic coefficient of friction is small even under bad environmentalconditions, the composite materials are clearly very promising materialsas braking members in braking devices such as of aircraft which requirehigh reliability even under bad conditions. Particularly, the compositematerials have two kinds of pores differing in pore diameter, thesurface roughness increases, and is greater than that of the C/Ccomposite. Thus, they have extremely excellent properties as brakingmembers for mass-transportation means. Moreover, since the skeletal partcomprises the C/C composite, weight is light, loss of energy is small,and demand for energy savings is met.

Further, since the skeletal part comprises the C/C composite, thecomposite materials are high in tenacity, hardness and impactresistance. Therefore, they exhibit the effect to increase kineticcoefficient of friction for mass-transportation means used at hightemperatures and in the presence of oxygen. Moreover, as shown in FIG.5, the composite materials have a high temperature of 757° C. at whichthe weight reduces by 5%, said temperature being an indicator foroxidation characteristics, and have high high-temperature resistancecharacteristics. Therefore, the composite materials are useful as moltenmetal members used under conditions exposed to high temperature. Forexample, since in the molten metal pump of the present invention, atleast the portions which contact with molten metal are made of theSiC—C/C composite materials having excellent thermal impact resistanceand oxidation resistance, even when they are used in a molten metal ofhigh temperature for a long time, they do not crack or are not oxidized,and the life of the molten metal pump can be prolonged. As a result,production cost for plated articles can be lowered. Furthermore, sincesilicon hardly dissolves into the molten metal, reduction of platingpurity of the plated articles caused by the dissolved silicon can beinhibited.

What is claimed is:
 1. A method for producing an SiC—C/C compositematerial comprising silicon carbide, carbon fibers and a carboncomponent other than the carbon fibers and having a structure comprisinga skeletal part and a matrix formed around the skeletal part, at least50% of silicon carbide being of β type, the skeletal part being formedof carbon fibers and a carbon component other than the carbon fibers,silicon carbide capable of being present in a part of the skeletal part,the matrix being formed of silicon carbide, the matrix and the skeletalpart being integrally formed, and the composite material having aporosity of 0.5-5% and a two-peak type distribution of average porediameter, said method comprising a step of keeping metallic silicon anda molded body comprising a C/C composite or a C/C composite fired bodyin a furnace at a furnace inner temperature of 1100-1400° C. and under afurnace inner pressure of 0.1-10 hPa for 1 hour or more with flowing aninert gas in an amount of 0.1 NL or more per 1 kg of total weight of themolded body or the fired body and the metallic silicon, thereby reactingthe carbon component constituting the matrix of the C/C composite withthe metallic silicon to form a matrix comprising silicon carbide, a stepof raising the furnace inner temperature to 1450-2500° C. with keepingthe furnace inner pressure as it is, thereby melting and impregnatingthe metallic silicon into open pores of the molded body or the firedbody to grow silicon carbide and simultaneously sufficiently filling theremaining pores with the metallic silicon, and a step of increasing thefurnace inner pressure to about 1 atm with once reducing the furnaceinner temperature to environmental temperature or keeping the furnaceinner temperature as it is, and raising the furnace inner temperature to2000-2800° C., whereby the produced silicon carbide is diffused from thematrix into the C/C composite composed of the carbon fibers and a carboncomponent other than carbon fibers and is reacted with the carbon.
 2. Amethod for producing an SiC—C/C composite material according to claim 1,wherein open pore content in the vicinity of the surface of the C/Ccomposite used is 5-50%.